Rift Valley fever (RVF) is a serious disease caused by a member of the Bunyaviridae family. RVFV is transmitted by mosquitoes and provokes disastrous outbreaks. Infections in humans can lead to hepatitis or encephalitis and fatal hemorrhagic fever;ruminants are also very sensitive. There is no commercial vaccine for humans and the Smithburn attenuated vaccine available for animals is teratogenic and abortogenic. Animal studies as well as biochemical analysis of infected cells have shown that the interferon response plays a critical role in attenuation but is blocked during infection with virulent strains. The nonstructural protein NSs is responsible for this blockage which occurs at the transcriptional level. To disable the innate immune evasion, NSs appears therefore as a good target. Our working hypothesis is based on the finding that interferon antagonistic action of NSs occurs via its interaction with p44 a component of the transcription factor TFIIH. Thus we propose to produce recombinant RVFV by reverse genetics in which the NSs function is abolished through NSs mutations. The proposed project is composed of three aims: i) establishment of a minigenome rescue system as a basis for the development of reverse genetics applied to RVFV, ii) production of a cDNA infectious clone system for the production of a recombinant virus and iii) production of a RVFV in which mutations in the interacting domain of NSs and p44 are introduced, the domain being determined concomitantly. The pathogenicity of this virus will be tested in the established mouse animal model to determine if the interaction of NSs with p44 is responsible for virulence. If the working hypothesis is true this virus must be avirulent. This will provide us with potential targets to develop antiviral strategies. In addition we will produce a recombinant virus which does not express any NSs protein and which must be a good vaccine candidate. In conclusion, the lack of control and prevention measures, the continued emergence of the virus in different parts of the world, and the omnipresent threat of bioterrorism make dissection of RVFV gene functions and investigation of RVFV pathogenesis a particularly urgent task. A vaccine would be extremely useful in future eradication programs by reducing infection in cattle, sheep and humans, and reducing the social, economic, and environmental impact of the disease.